During aging, signals from the thymic microenvironment required for normal T cell production are lost or dysregulated in a process called thymic involution. Prior mechanistic studies of thymic involution have focused largely on changes that occur in aged mice. The overall goal of our proposal is to understand how interactions between the components of the thymic microenvironment drive the profound and rapid transition in thymic growth and function from a neonatal to an adult phenotype, prior to the onset of involution. We reason that pathways that support the young growing thymus will provide new targets for rejuvenation of aged thymus. This proposal will build on our previous studies of thymic development during the transitional neonatal period, and will focus on cross-talk mechanisms between the non-hematopoietic compartments of the thymus, i.e. thymic epithelial cells (TECs), thymic mesenchymal cells (TMCs) and the vascular niche. We have previously demonstrated that, in neonates, high levels of Vascular Endothelial Growth Factor (VEGF) support angiogenesis and survival of perivascular cells (pericytes), and also indirectly affect the proliferation and maturation of TECs and thymocytes. Through the refinement of methods to isolate viable cells from each rare stromal subpopulation from fetal, neonatal and young adult thymus, we have been able to use high throughput RNA-Sequencing to generate a unique transcriptional profile of thymic stromal development. In addition, we have developed a model in which we can manipulate the stromal elements of human thymus in vitro and in vivo allowing us to translate findings in our mouse studies to assess their relevance to human thymic development. Our proposal has a large discovery element that will further refine our transcriptional profiling of stromal subpopulations isolated from murine and human thymus, and will apply advanced computational methods to discover novel pathways relevant to thymic growth and involution. We will now bring these diverse experimental approaches to bear on the goals of this proposal: to uncover novel ligand-receptor interactions between the mesenchymal and epithelial compartments of the thymus that regulate organ growth and maturation, to determine if these interactions are VEGF-dependent, and to test if these factors are active on human TECs. These goals are designed to move us toward clinically relevant targets for rejuvenating thymic function. We also anticipate that our studies will reveal novel pathways and cellular interactions that are broadly shared in other epithelial tissues during development, aging and rejuvenation.